Activity-dependent Fos-like expression was investigated immunohistochemically in rainbow trout (Oncorhynchus mykiss) that had performed vibratory-evoked startle responses. We found significantly higher numbers of Fos-like-immunoreactive neurons in the reticular formation, in the octavolateral area, and in several cranial nerve motor nuclei in the brain and in the motor column of the spinal cord of startled fish than in control fish. In one fish, in which stimulation did not evoke startle responses, substantial numbers of positive cells occurred in the brain, primarily in the magnocellular octavolateral nucleus. We observed Fos-like-immunoreactive neurons in cell groups that are known to participate in the startle response (e.g., the Mauthner cell) as well as in cell groups that have been proposed but until now not shown to be involved. J. Comp. Neurol. 439:306–314, 2001. © 2001 Wiley-Liss, Inc.